Semiconductor based light emitting diodes (LEDs) are among the most efficient light sources available today. LEDs provide longer lifetime, higher photon flux efficacy, lower operating voltage, narrow-band light emission, and flexibility in terms of assembly compared to conventional light sources.
GaN based LEDs are for instance successfully employed as high power light sources in solid state lighting applications such as illumination, traffic lighting, indoor/outdoor displays, and backlighting electronic displays.
It is however challenging to efficiently couple light out of the LEDs. An epitaxial layer normally has a high refractive index compared to air, nair=1, or glass, nglass=1.5. The refractive index of GaN, nGaN, is for example in the range 2.3-2.5 at visible wavelengths. The large refractive index mismatch between the epitaxial layer and a surrounding medium causes a large portion of the light generated within the epitaxial layer to be scattered or reflected at the interface between the epitaxial layer and its surrounding medium. Only light travelling at angles within a relatively narrow escape cone associated with the interface can refract into the surrounding medium and escape the epitaxial layer. In other words, the external quantum efficiency of the LED is low resulting in a reduced brightness of the LED.
The situation is even more complex when it comes to white light generation using LEDs. White light is generally not generated by tailoring of the emission energy of the epitaxial layer. Instead, light from a blue epitaxial layer, typically GaN based, is converted to white light with the aid of a yellow phosphor material surrounding the epitaxial layer. The surrounding phosphor material down-converts a substantial portion of the epitaxial layer's blue light, changing its color to yellow. Hence, the LED emits both blue and yellow light, which in combination provide white light. In another approach light from a violet or ultraviolet emitting epitaxial layer has been converted to white light by surrounding the epitaxial layer with multicolor phosphors.
The phosphor material, which is typically deposited on a substrate on which the epitaxial layer is grown, causes additional light losses due to scattering at the interface between the substrate and the phosphor material as well as back scattering of light at for instance an air/phosphor material interface.
In order to improve the performance of the LEDs there is therefore a need for better efficiency in guiding light emitted from the epitaxial layer out from the LED.